The present invention relates to a ZrO2 porcelain and a waterproof ZrO2 member which are to be used as members of a fluid sensor or the like
A ZrO2 porcelain is widely used as a material comprising an vibrating sheet which is used for disposing a piezoelectric/electrostrictive element in a fluid sensor. The fluid sensor usually comprises a sensor unit which consists of a plate like member 2 having a piezoelectric/electrostrictive element 10 mounted thereon and a cylindrical member 1 which supports the sensor unit, for example, as shown in FIG. 5. The fluid sensor is configured to allow a sample fluid to be introduced into the cylindrical member 1 and used to measure, for example, a degree of deterioration of a battery by detecting a viscosity, density of the electrolyte or a specific compound in the electrolyte. Both the plate like member 2 and the cylindrical member 1 are made of ZrO2. Ordinarily used as such, a ZrO2 is a partially stabilized ZrO2 which contains 3 to 4 mol % of yttria.
However, the ZrO2 which contains 3 to 4 mol % of yttria poses a problem that it is low in long-term durability and reliability, though it is sufficient in its strength. Speaking concretely, it poses problems described below when it is kept in contact with a liquid or a gas which contains a liquid vapor for a long term, for example, in a condition used as a member of a fluid sensor.
When the fluid sensor is used to evaluate deterioration of a battery, for example, it evaluates a degree of deterioration by utilizing a definite relationship of a viscosity, density of a deteriorated electrolyte of the battery versus a concentration of sulfuric acid and a fact that a concentration of sulfuric acid changes in an electrolyte as the battery is deteriorated. In this case, an acid solution used as the electrolyte is kept in direct contact with the vibrating sheet and the cylindrical member. The vibrating sheet and the cylindrical member may be eroded when they are kept in contact with the acid solution continuously for a very long term, for example, several to some tens of years though no problem is posed in use for a relatively short term. The inventor et al. conducted accelerated tests by keeping vibrating sheet of ZrO2 porcelains in continuous contact with 40% sulfuric acid at 80xc2x0 C. and found a problem that a porcelain containing yttria at 3 mol allowed the sulfuric acid to leak to an opposite surface of the surface where sulfuric acid contact in Five to thirty days and a porcelain containing yttria at 4 mol allowed the sulfuric acid to leak to a side opposite to an vibrating sheet in ten to Forty days.
It is considered that the leakage is traced to gradual transformation of a tetragonal phase, out of the ZrO2 crystalline structures, into a monoclinic phase which is caused when ZrO2 is left standing in a liquid or a gas containing a liquid vapor.
Though it can be considered to change the crystalline structure of ZrO2 into a cubic crystal which is stable in a liquid or a gas containing a liquid vapor in order to avoid such inconvenience, the cubic crystal has low toughness and is fragile, thereby posing a problem that cracks 11 are formed as shown in FIG. 6 at portions of an integral structure consisting of an vibrating sheet and a cylindrical member on which stresses are concentrated.
Furthermore, there is available a method which makes a ZrO2 porcelain stable in a liquid or a gas containing a liquid vapor by reducing a particle size of a raw material powder or using an additive, such as magnesia or alumina. However, this method poses a problem that it makes stability of the ZrO2 porcelain insufficient and requires high temperature to sinter a piezoelectric/electrostrictive element which is disposed on a vibrating sheet, thereby enlarging the particle size and lowering the stability.
The present invention has been achieved in view of the circumstances described above and has an object to provide a ZrO2 porcelain which is not only stable but also tough, can hardly be eroded and has sufficient strength in a liquid or a gas containing liquid vapor.
That is, the present invention provides a ZrO2 porcelain comprising a ZrO2 porcelain flexible member having at least a part which is brought into contact with a liquid or a gas containing a liquid vapor,.wherein the contact part has a relative density (bulk density/theoretical density) not lower than 95% and contains a cubic crystal at a ratio not lower than 75%.
In the ZrO2 porcelain, it is preferable that the ZrO2 flexible member consists of a contact part and a substrate adjacent to the contact part, and that the substrate is composed of a tetragonal crystal or a mixture phase consisting of a cubic crystal and a tetragonal crystal or a cubic crystal, a tetragonal crystal, and a monoclinic crystal. Furthermore, it is preferable that the contact part covers at least one surface of the substrate and that a thickness of the contact part is 0.2 to 2 times of that of a thinnest portion of the substrate.
The ZrO2 porcelain may consist of a cylindrical member made of a ZrO2 porcelain and a ZrO2 porcelain flexible member which is composed by disposing a contact part at least on one surface of a plate like member consisting of the substrate, and having an integral structure obtained by integrating the ZrO2 porcelain flexible member with the cylindrical member which is a supporting member for the flexible member so that an end surface of the cylindrical member is brought into contact with the contact part disposed on the plate like member and the contact part closes an opening of the cylindrical member.
Furthermore, the ZrO2 porcelain may consist of a cylindrical member made of a ZrO2 porcelain and a ZrO2 porcelain flexible member which is composed by disposing a contact part at least on one surface of a plate like member consisting of the substrate, comprise a connecting layer which is formed on at least one end surface of the cylindrical member and has a configuration which is the same as that of the contact part, and having an integral structure which is obtained by integrating the ZrO2 porcelain flexible member with the cylindrical member which is a supporting member for the flexible member so that the connecting layer is brought into contact with the contact part disposed on the plate like member and the contact part closes an opening of the cylindrical member.
Furthermore, the ZrO2 porcelain may consist of a cylindrical member made of a ZrO2 porcelain and a ZrO2 porcelain flexible member which is a plate like member consisting of the contact part, and have an integral structure obtained by integrating the ZrO2 porcelain flexible member with the cylindrical member which is a supporting member for the flexible member so that an end surface of the cylindrical member is brought into contact with the plate like member and the contact part closes an opening of the cylindrical member.
Furthermore, the ZrO2 porcelain may consist of a cylindrical member made of a ZrO2 porcelain and a ZrO2 porcelain flexible member which is a plate like member consisting of the contact part, comprise a connecting layer which is formed on at least one end surface of the cylindrical member and has the same configuration as that of the contact part, and have an integral structure obtained by integrating the ZrO2 porcelain flexible member with the cylindrical member which is a supporting member for the flexible member so that the connecting layer is brought into contact with the plate like member and the contact part closes an opening of the cylindrical member.
For the ZrO2 porcelain according to the present invention, it is preferable to dispose a contact part which has a relative density (bulk density/theoretical density) not lower than 95% and contains a cubic crystal at a ratio not lower than 75%, on inside walls of a space formed by the ZrO2 porcelain flexible member and the cylindrical member which is a supporting member for the flexible member.
The ZrO2 porcelain according to the present invention may comprise a cylindrical member made of a ZrO2 porcelain; and a plate like member composed of the substrate, wherein an end surface of the cylindrical member is in contact with the plate like member, a contact part which has a relative density (bulk density/theoretical density) not lower than 95% and contains a cubic crystal at a ratio not lower than 75% is disposed on inside walls of a space formed by closing an opening of said cylindrical member, and a ZrO2 porcelain flexible member is integrated with the cylindrical member which is a supporting member for the flexible member.
In the ZrO2 porcelain according to the present invention, the cylindrical member may be composed of a substrate which consists of a tetragonal crystal or a mixture phase consisting of a cubic crystal and a tetragonal crystal or a cubic crystal, a tetragonal crystal, and a monoclinic crystal, the cylindrical member may be composed of a contact part which has a relative density (bulk density/theoretical density) not lower than 95% and contains a cubic crystal at a ratio not lower than 75% or the cylindrical member may be composed of an intermediate layer which contains a cubic crystal at a ratio higher than the substrate and lower than the contact part. Furthermore, the ZrO2 porcelain according to the present invention may have an integral structure of a cover member and a cylindrical member wherein the cover member is attached to the other end surface of the cylindrical member so as to close an opening of the cylindrical member and a hole formed in the cover member communicates with an internal space of the cylindrical member. In this case, the cover member may be composed of a substrate which consists of a tetragonal crystal or a mixture phase consisting of a cubic crystal and a tetragonal crystal or a cubic crystal, a tetragonal crystal and a monoclinic crystal. In the ZrO2 porcelain according to the present invention, an intermediate ZrO2 layer which contains a cubic crystal at a ratio higher than the substrate or the cylindrical member and lower than the contact part or the connecting layer may be disposed at least at a location between the substrate or the cylindrical member and the contact part or the connecting layer, and it is preferable that the intermediate ZrO2 layer contains a cubic crystal at a ratio not lower than 60% and not higher than 95%. The ZrO2 porcelain according to the present invention may comprise a piezoelectric/electrostrictive element which is disposed on a surface of the ZrO2 porcelain flexible member. Furthermore, the ZrO2 porcelain according to the present invention may contain lead.
Moreover, the present invention provides a fluid sensor which uses the ZrO2 porcelain described above. Furthermore, the present invention provides a fluid sensor comprising: a piezoelectric element having a piezoelectric film and at least a pair of electrodes disposed in contact with said piezoelectric film, on the opposite surface of the surface where said cylindrical member is located, of said ZrO2 porcelain flexible member composing the ZrO2 porcelain described above, electrode terminals disposed on the opposite surface of the surface where said cylindrical member is located, of said ZrO2 porcelain flexible member and electrically connected to said pair of electrodes; a supporting member fixing said ZrO2 porcelain flexible member; and a separating member maintaining said piezoelectric element and said supporting member in contactless conditions; wherein said separating member is disposed on the opposite surface of the surface where said cylindrical member is located, of said ZrO2 porcelain flexible member so as to surround said piezoelectric element, and said supporting member is in contact with said ZrO2 porcelain flexible member and fix it by way of said separating member, characterized in that a glass layer is formed on the opposite surface of the surface where said cylindrical member is located, of said ZrO2 porcelain flexible member which is kept in contact with said separating member.
In addition, the present invention provides a waterproof ZrO2 member to be used in an atmosphere containing water or water steam, wherein a substrate is made of ZrO2 which is is composed of a tetragonal crystal or a mixture phase consisting of a cubic crystal and a tetragonal crystal or a cubic crystal, a tetragonal crystal, and a monoclinic crystal, and a ZrO2 layer which has a relative density (bulk density/theoretical density) not lower than 95% and contains a cubic crystal at a ratio not lower than 75% is disposed at a location to be exposed to an atmosphere containing water or water steam. The atmosphere containing water or water steam may be an acidic atmosphere.